A large number of flow tubes which raw material of paper (stock) flows through are arranged in the tube bank of the headbox of a papermaking machine for stabilizing the flow therethrough of stock and conducting it to nozzles provided at downstream portion of the tubes. The stabilized flow of the stock at the outlet of the tube bank is important and a variety of arts are put into practice to improve the stabilizing effect in the tube bank to suppress turbulent flow of the stock at the outlet of the tube bank as the machine becomes increasingly faster.
One of improved arts is disclosed in Japanese Patent Publication No.6-506274, the front view of the tube bank of which is shown in FIG. 7.
In this invention, a plurality of flow tubes 2 arranged in the tube bank of the headbox of the papermaking machine are formed such that the inlet side portion of circular cross-section of each flow tube is joined to a larger diameter circular pipe portion of a diameter larger than that of said inlet side portion and the larger diameter circular pipe portion is succeeded by a portion of which the configuration of cross-section is smoothly changed to a rectangular section at the outlet opening end while maintaining the circumferential length constant.
In the invention of Japanese Patent Application Publication No.56-148990, a plurality of flow tubes, each tube being formed such that the inlet side cross-section is of circular configuration and the configuration of section changes smoothly while increasing in cross-sectional area to a rectangular configuration at the outlet opening end, are arranged such that said outlet opening ends are aligned in staggered arrangement.
However, there are problems in said prior arts as follows:
In the invention of Japanese Patent Publication 6-506274, the flow tube 2 is formed such that the circular inlet side portion is joined to a larger diameter circular pipe portion of a diameter larger than that of said inlet side portion and the larger diameter circular pipe portion is succeeded by a portion of which the configuration of cross-section is smoothly changed to a rectangular section at the outlet opening end and a plurality of the flow tubes are arranged such that the rectangular outlet opening ends are aligned in a vertical direction, so that the center distance between adjacent rectangular outlet opening ends in the vertical direction is defined by the outer diameter of said larger diameter circular pipe portion. Therefore, there is formed between adjacent outlet opening ends a rand S of large width corresponding to the length (diameter of said larger diameter pipe portion−width(shorter side) of the rectangle of outlet opening end+2 times of wall thickness of the flow tube 2).
As the rands S are formed on the rectangular outlet end parts of the flow tubes in the prior art as mentioned above, turbulence of large size is developed immediately downstream relative to the rectangular outlet end parts. To prevent the development of turbulence, a flow seat is provided for stabilizing the flow of stock immediately downstream relative to each rectangular opening end, which results in complicated structure and poor maintainability because of the necessity of removing the flow seat when performing maintenance servicing.
In the invention of Japanese Patent Publication No.56-148990, each of the flow tubes is formed such that it is increased in diameter from the inlet opening end toward the outlet opening end and the inlet end of each of the inlet opening end is welded to a plate while aligning a plurality of the flow tubes, so the manufacturing cost of the flow tube bank is increased. Further, to complement the reduction in dispersion effect because of the absence of a stepped part, the length of the flow tube is formed to be very long, which may induce to the development of concentration streak after the stock flow outflows the flow tube. To prevent the development of concentration streak, the rectangular outlet openings of a plurality of said flow tubes are aligned in staggered arrangement. Therefore, flow tubes different in shape and cross-sectional area are to be located in both ends in the direction of width of the tube arrangement. As a result, dispersion and flow rate in the center part are different from those in the both end parts of the flow passage.